Normally, the dozing operation of a bulldozer of the above type is carried out under manual control by the operator. Concretely, the blade is manually controlled so that digging or soil carrying is performed with the blade being lifted and lowered, or leveling is performed with the cutting edge of the blade kept in a certain position in relation to the ground.
In such manual operation to lift or lower the blade, or keep the position of the cutting edge, the operator is required to frequently manipulate the blade so that he gets tremendous fatigue, no matter how skillful he is. In addition, such manipulation is too complicated for an inexperienced operator.
As an attempt to solve this problem, the applicant of the present invention has proposed a leveling control system for a bulldozer in Japanese Patent Publication (KOKAI) No. 7-48855 (1995), which enables leveling work in dozing operation by simple manipulation without causing extreme fatigue. In this leveling control system, a lift operation amount is obtained from a load control characteristic map to make the actual tractive force of the bulldozer equal to a target tractive force, while a lift operation amount is obtained from a leveling control (smoothing control) characteristic map to make the actual position of the cutting edge relative to the ground coincident with a target cutting edge position. These lift operation amounts are respectively weighted with a value obtained from a load-leveling control weight characteristic map, based on the difference between the actual and target tractive forces and then summed, in order that a final lift operation amount is obtained.
The leveling control system of this publication, however, presents the following problem. In this system, even when the load exerted on the blade is greatly changed, a target value for the load control is corrected by a target value for the smoothing control. Therefore, upon completion of carrying operation for example, the load control is so performed as to lift the blade, whereas the smoothing control is so performed as to lower the blade to restrict the fluctuation of the target cutting edge position. Consequently, the resultant ground surface after dozing operation will be undulated.
In addition, according to the publication, the load-leveling control weighting characteristic map is always set based on a constant weight function notwithstanding changes in the working states of dozing operation, and therefore the weight function of such a map is inevitably a combination of a weight function for digging work and a weight function for carrying work. This poses an obstacle to improvements in control performance.
The above publication has a further disadvantage in that when performing digging work and carrying work a plurality of times in the same lane, a target value is reset for every cycle of dozing operation so that improvement cannot be expected from the repetitive cycles and consequently there remain difficulties in adjusting the dozing operation to soil property and working conditions which vary every excavation site.
The invention is directed to overcoming the above problems and a prime object of the invention is therefore to provide a dozing system for a bulldozer, which provides improved operational efficiency for dozing operation while achieving a smooth excavation face. A second object of the invention is to provide a dozing system for a bulldozer, which is capable of adequately setting a weight function according to whether a digging mode or carrying mode is presently selected, thereby achieving better control performance. A third object of the invention is to provide a dozing system for a bulldozer, which exhibits good conformability to variations in the conditions of every excavation site to thereby achieve improved operational efficiency.